Sellafield has been fined £75,000 and ordered to pay costs of £26,100 after two contractors were exposed to radiation at the nuclear site in Cumbria.
Two men working for Workington building company Stobbarts were drilling through a concrete floor contaminated with plutonium when they inhaled radioactive dust on 11 July 2007.
Sellafield pleaded guilty to two breaches of the Health and Safety at Work Act at an earlier hearing before it was sentenced at Carlisle Crown Court today.
The incident was called "a regrettable episode" by Judge Paul Batty, but he did accept the company had an impressive track of safety and had implemented measures to ensure to try to avoid similar incidents in the future.
Sellafield, now owned by Amec, Areva of France and URS Washington of America, was accused of a catalogue of safety errors including failing to assess the risks at the site where the men were working, failing to train them properly and failing to provide adequate safety equipment.
The Health and Safety Executive said the dose of radiation they were exposed to, however, was below the annual dose limit and there was no immediate impact on their health.